The past couple of weekends I've been able to do some carb testing on a local dyno using some of the things I've learned here. The engine is small, 357 inches, 13:1 compression, roller cam, etc. All spare parts the dyno operator had in the shop. It's a mutt motor. Yes, I used Dominators, mine, and made a max of 511HP. I've invested in this engine and the sole purpose here is my education in carbies. Over the course of a week we made 28 pulls.

So, we had a fuel curve that got progressively rich all the way to 7600 RPM. I was able to to flatten that out by adding emulsion in the lower holes area of the mainwell. Just what I expected to happen. It's also the opposite of what happens with the same e-hole tune in my 540. I'm wondering why the difference. This was the same carb but the jetting was reduced as needed.

The very last pull was made with a main section, no other changes, made for me by BLP. This had a 1.780 venturi and a 2.125 throttle plate. There was a 10 HP increase.

So, we had a fuel curve that got progressively rich all the way to 7600 RPM. I was able to to flatten that out by adding emulsion in the lower holes area of the mainwell. Just what I expected to happen. It's also the opposite of what happens with the same e-hole tune in my 540. I'm wondering why the difference. This was the same carb but the jetting was reduced as needed.

My way of tuning would have been to open up the MABs to try to lean it down up top. I've had that work on more than one tuneup? Adding additional e holes still is not "likely" to flow any more air than the MAB will allow.

nomad wrote:

The very last pull was made with a main section, no other changes, made for me by BLP. This had a 1.780 venturi and a 2.125 throttle plate. There was a 10 HP increase.

Are these dimensions the same as the ones in the Holley body you started with?

Ken, by adding emulsion I meant increasing the size of an existing hole. #4 in a BLP block. On the main body change the only difference was the main body itself. Same tuneup.

I learned a lot. Like run new headers to burn off all the paint before you stick a new O2 sensor in them.

BTW Ken, Normally I would have started with a MAB change but, but I wanted to see the effect of going from an .028 to an .029. The high RPM range became leaner and flattened the curve, and, made the overall curve a little bit leaner.

Last edited by nomad on Wed Jul 20, 2011 5:58 pm, edited 1 time in total.

The past couple of weekends I've been able to do some carb testing on a local dyno using some of the things I've learned here. The engine is small, 357 inches, 13:1 compression, roller cam, etc. All spare parts the dyno operator had in the shop. It's a mutt motor. Yes, I used Dominators, mine, and made a max of 511HP. I've invested in this engine and the sole purpose here is my education in carbies. Over the course of a week we made 28 pulls.

So, we had a fuel curve that got progressively rich all the way to 7600 RPM. I was able to to flatten that out by adding emulsion in the lower holes area of the mainwell. Just what I expected to happen. It's also the opposite of what happens with the same e-hole tune in my 540. I'm wondering why the difference. This was the same carb but the jetting was reduced as needed.

The very last pull was made with a main section, no other changes, made for me by BLP. This had a 1.780 venturi and a 2.125 throttle plate. There was a 10 HP increase.

Next up, some E-85 carbs.

hey fellas,

im chasing a lean early / rich late progression myself. i have read tuners thread on emulsion theory....good stuff but i need to see it on a chalk board was hoping for some real world info to support some "changes" i can make to lean out the top of our curve ( or richen the bottom). basically flatten the curve.12.6 afr early up to 6000 rpm.....trending rich 12.1 late up to 7800 rpmit has been suggested that i enlarge my bottom e-hole to .040 dia. (currently .032)IAB is at .076 to work with an .040 IFR and a .052 trans slot restrictor.

So, we had a fuel curve that got progressively rich all the way to 7600 RPM. I was able to to flatten that out by adding emulsion in the lower holes area of the mainwell. Just what I expected to happen. It's also the opposite of what happens with the same e-hole tune in my 540. I'm wondering why the difference. This was the same carb but the jetting was reduced as needed.

The reason is because you changed the main jet. The effect of the e-bleeds depends upon the head of pressure of fuel in the main well and the main jet controls that.

Scientific Researchers have proved locating the booster where I said is the best design. I have no idea why other people do things the way they do. Perhaps they need to read more. Carburetors and atomization etc are a very well researched subjects with on going modern day effort even in space. Carbys are not finished yet

Either or any system will work. What the designer is after is a supply of Air to the engine and then a ratio of fuel delivered. You can do that many ways. It doesn't matter what efficiency the venturi is as long as its able to flow the air required and a suitable emulsion system can be implemented in the space available.A perfectly designed venturi can achieve a pressure recovery at the outlet of about 98% efficient, a poorly designed one will be around 94% and a stupid ridiculous one can get as low as in the 70's% range. It doesnt matter if your venturi is any of these designs. All the designer is looking for on stock engines is a maximum vacuum rating in the manifold that will assist the engine to run with sufficient fuel vaporization to reduce bore wear. OEM's cant have their engine wearing out when you start towing loads etc. They compete with fuel efficiency marketing etc. The best way to get an engine fuel efficient and durable and stop it from overheating and requiring larger expensive radiators and stop the exhaust from burning everything is to have high vaporization when its running high loads. The best way to do that is to increase the manifold vacuum at high loads. You do that by running a smaller carby. If you want more power then you build a bigger cubic capacity engine. More cubes but less power per cubic inch is a solution to all those problems. Its a cheap method.

The nozzle venturi in your example is a typical carby one, its not optimum at efficiency of pressure recovery but it doesn't matter because they just make the hole bigger. It cheaper to make and easier to get accuracy and its shorter so it fits under the bonnet. None of the common carby stuff that we buy is designed for the science. If they poke the end of the booster past the venturi or into the venturi all they do is restrict the CFM of the engine so what doesn't that matter? They can build any size hole they want to feed the engine.